A conventional portable lifting jack normally comprises a wheeled frame, a lifting arm retractably received in the frame, a hydraulic pump for driving the lifting arm and a handle connected to the hydraulic pump. A hydraulic cylinder is disposed at a front side of the hydraulic pump. A hydraulic shaft is extendable from and retractable in the hydraulic cylinder. A rear end of the hydraulic shaft is movably fitted in the hydraulic cylinder. A front end of the hydraulic cylinder is connected with the lifting arm via a link arm.
The hydraulic pump generally comprises a multipurpose block having at least one oil chamber and a plurality of oil channels defined therein, a releasing device disposed in a release bore defined in a rear side of the multipurpose block, a piston cylinder disposed in a piston bore defined in the rear side of the multipurpose block, and a hydraulic cylinder securely connected with the multipurpose block at a threaded hole defined in a front side of the multipurpose block. When a user swings the handle manually, the lifting arm is driven by the hydraulic pump to move pivotally between a horizontal direction and an inclined direction. One prior art portable lifting jack is shown in U.S. Pat. No. 4,018,421.
However, the conventional portable lifting jack still has a defect in that during operation of the portable lifting jack under a load, the oil chamber of the hydraulic pump often reaches a sub-atmospheric pressure. In order to avoid the hydraulic pump producing such a sub-pressure in the operation, the oil chamber of the hydraulic pump normally has some air in addition to the oil, which causes a decrease of the efficiency of the hydraulic pump. Therefore, it is an objective of the invention to provide an improved portable lifting jack to mitigate and/or eliminate the aforementioned problems.